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John C. Walker

Director of Biological Sciences
Curators Professor of Biological Sciences
PhD, 1985 University of Georgia

Email: walkerj@missouri.edu
Office: 321 Tucker Hall
Phone: 573-882-3583
Additional: Website
Headshot of John C. Walker

Research

Research summary

Molecular mechanisms regulating cellular signaling in plants

Research description

TPJ Front Cover:<br>

Precise regulatory mechanisms are necessary for the proper control of enlargement and patterning of plant lateral organs. For example, these regulatory mechanisms are responsible for ensuring proper flower development. In the May 2009 issue of <I>The Plant Journal Larue et al</I> (58(3):450-463) report the identification of a novel allele of the NAC domain transcription factor <I>CUC2, cuc2-1D </I> which carries a mutation in the  <I>CUC2 </I> microRNA targetsite, disrupting microRNA targeting. <I>cuc2-1D </I>plants have enlarged vegetative and reproductive lateral organs with disrupted patterning relative to wild-type plants. The data presented suggests that <I> miRNA164 </I> (the <I> CUC2 </I> targeting miRNA) and <I> CUC2</I>  form a central regulatory module through with both lateral organ patterning and expansion are governed. This figure shows false-colored scanning electron micrographs of two partially dissected <I> cuc2-1D</I>  flowers. These show a variety of developmental abnormalities, including enlarged components and disrupted patterning. The enlarged flower on the left illustrates a young developmental stage before the flower has opened, while the flower on the right illustrates a mature flower that has fully opened. 
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<a href=http://www3.interscience.wiley.com/journal/121641419/abstract>View  abstract </a>

TPJ Front Cover:
Precise regulatory mechanisms are necessary for the proper control of enlargement and patterning of plant lateral organs. For example, these regulatory mechanisms are responsible for ensuring proper flower development. In the May 2009 issue of The Plant Journal Larue et al (58(3):450-463) report the identification of a novel allele of the NAC domain transcription factor CUC2, cuc2-1D which carries a mutation in the CUC2 microRNA targetsite, disrupting microRNA targeting. cuc2-1D plants have enlarged vegetative and reproductive lateral organs with disrupted patterning relative to wild-type plants. The data presented suggests that miRNA164 (the CUC2 targeting miRNA) and CUC2 form a central regulatory module through with both lateral organ patterning and expansion are governed. This figure shows false-colored scanning electron micrographs of two partially dissected cuc2-1D flowers. These show a variety of developmental abnormalities, including enlarged components and disrupted patterning. The enlarged flower on the left illustrates a young developmental stage before the flower has opened, while the flower on the right illustrates a mature flower that has fully opened.
View abstract

Multicellular organisms need to carry out many processes in a coordinated manner to sense and respond to both external and internal signals in an intricate and precise way. Multi-step signal transduction creates the necessary complexity for refined regulation of a cell's response to developmental signals and its environment. A common way cells relay molecular messages is by reversible protein phosphorylation; protein kinases add phosphates to their target protein(s) and protein phosphatases remove them. Cells can begin this process with receptor protein kinases, using phosphorylation status to transduce external messages into the cell. A plethora of candidate receptor protein kinases have been found in plants, but only a few have been investigated. Function for the plant receptor-like kinases (RLKs) has been shown in various biological processes such as development, disease resistance and self-incompatibility.

There are several hundred RLK genes and they represent the largest group of cell surface receptors in plants. The 1999 report of the NSF-Sponsored Workshop: New Directions in Plant Biological Research points out the RLKs represent unexpected discoveries derived from the Arabidopsis genomic sequence. The report states, "What are the roles of the hundreds of these proteins? Their existence implies a massive network of cell-cell and environment-plant communication, via a series of ligands yet to be discovered. Understanding this network will give us an entirely new view of plant development, environmental response, and organismal integration”. Our research is directed toward understanding the function of the RLK genes.

We are using the approaches of functional genomics and proteomics to understand RLK function. We have ongoing projects to 1) Isolate and characterize loss-of-function and gain-of-function mutations; 2) Determine the patterns of expression of the mRNAs, and establish the cellular and subcellular locations of these receptors by use of green fluorescent protein fusions; 3) Define the regulatory networks that mediate signaling by these protein kinases by screening for genetic modifiers and interaction partners. Although we are not yet able to describe an entire signal transduction cascade for any one RLK, these approaches promise to provide important insights into the molecular mechanisms by which the receptor protein kinases control development and adaptive responses in plants.

Select Publications

Select Publications

Taylor I, Wang Y, Seitz K, Baer J, Bennewitz S, Mooney BP, Walker JC: Analysis of Phosphorylation of the Receptor-Like Protein Kinase HAESA during Arabidopsis Floral Abscission. PLoS ONE 2016, 11(1).

Patharkar OR, Walker JC: Floral organ abscission is regulated by a positive feedback loop. Proceedings of the National Academy of Sciences of the United States of America 2015, 112(9):2906-2911.

Li J, Hou J, Sun L, Wilkins JM, Lu Y, Niederhuth CE, Merideth BR, Mawhinney TP, Mossine VV, Greenlief CM et al: From gigabyte to kilobyte: A bioinformatics protocol for mining large RNA-Seq transcriptomics data. PLoS ONE 2015, 10(4).

Niederhuth CE, Patharkar OR, Walker JC: Transcriptional profiling of the Arabidopsis abscission mutant hae hsl2 by RNA-Seq. BMC Genomics 2013, 14(1).

Valdivia ER, Hertweck KL, Cho SK, Walker JC: DVL/RTFL. In: Handbook of Biologically Active Peptides. 2013: 15-19.

Taylor I, Seitz K, Bennewitz S, Walker JC: A simple in vitro method to measure autophosphorylation of protein kinases. Plant Methods 2013, 9(1).

Niederhuth CE, Cho SK, Seitz K, Walker JC: Letting go is never easy: Abscission and receptor-like protein kinases. Journal of Integrative Plant Biology 2013, 55(12):1251-1263.

Meng X, Wang H, He Y, Liu Y, Walker JC, Torii KU, Zhang S: A MAPK cascade downstream of ERECTA receptor-like protein kinase regulates Arabidopsis inflorescence architecture by promoting localized cell proliferation. Plant Cell 2013, 24(12):4948-4960.

Valdivia ER, Chevalier D, Sampedro J, Taylor I, Niederhuth CE, Walker JC: DVL genes play a role in the coordination of socket cell recruitment and differentiation. Journal of Experimental Botany 2012, 63(3):1405-1412.

Honors & Awards

Selected honors and awards

Curators Professor, University of Missouri 2014

Excellence Award, Interdisciplinary Plant Group, MU 2012

Fellow, American Association for the Advancement of Science 2004

Chancellor’s Award for Outstanding Faculty Research and Creative Activity in Biological Sciences 1997